1. Field of Invention
This invention relates to the field of nuclear radiation measurement. Nuclear source strength is defined in terms of the number of atom disintegrations per unit of time. Typically, these disintegrations result in alpha (.alpha.), beta (.beta.), gamma (.gamma.), or neutron radiation fields. Detection and measurement of these field strengths has been the objective of radiation measurement apparatus, and detectors to sense and measure these fields traditionally have employed the interaction or absorption of energy from these fields. This invention describes apparatus for, and a method of, using a Geiger-Mueller tube (G-M) detector over a wide range of field strengths while circumventing the limitations traditionally encountered in the use of such devices at the high field regions of interest.
2. Prior Art
Detectors for sensing radiation fields have included ion chambers, proportional counters, Geiger Mueller tubes, scintillation crystals and solid-state semiconductors. Each has advantages and disadvantages, and selection of a detector has been affected by the particular application. A single measurement apparatus for measuring radiation fields from very low intensity to extremely high intensity is highly desirable, but has been difficult to achieve due to the limiting physics phenomena of the various detectors at either the high or low regions of interest.
G-M tubes have served as simple, rugged, dependable detectors. Traditionally, G-M tubes are biased with a high voltage to create a high field strength between the anode and cathode electrodes, which field strength is chosen to be below that which causes self-ignition, but high enough so that the interaction with a nuclear particle/wave is sufficient to result in a discharge within the tube. This discharge pulse is sensed and counted with appropriate support electronics. After the discharge, the electric field is restored and subsequent events can occur and similarly be counted. The rate of discharges is measured (counted) and used as a measure of the nuclear field strength.
Limitations occur at high fields since the G-M tube exhibits an inactive or dead time just following a discharge. This dead time limitation is further complicated by the fact that its extent is a function of the field strength. This phenomenon can actually result in complete crippling of the ability of a G-M tube to function at high fields. Dead time correction via electronics is a difficult task due to its non-linear character.